1. Field of the Invention
The embodiments of the invention generally relate to a metal oxide semiconductor field effect transistor (MOSFET) and to a method of forming the transistor with a sharp halo.
2. Description of the Related Art
Oftentimes halos are incorporated into metal oxide semiconductor field effect transistors (MOSFET) devices in order to reduce the short channel effects that have resulted from the scaling of such devices. Specifically, these halos have the same conductivity type as the transistor body and are generally positioned around the edges of the source/drain extensions below the gate. Such halos reduce the depletion region between the source/drain regions and the channel region and, thereby, reduce punch through (i.e., reduce lateral diffusion of dopants from the source/drain regions into the channel region). The strength of the halo effect depends on both the doping concentration of the halo and on confinement of the halo dopants (e.g., by limiting diffusion of the halo dopants towards the channel region and the substrate). Thus, by forming sharply defined halos that remain closely bound to the source/drain extensions, optimal punch through reduction can be achieved.
Traditionally, halos are formed by using a relatively high energy implantation process in order to penetrate the halo dopant (e.g., implantation of a first conductivity type dopant) to the correct depth. The halo implantation process is then followed by another implant (e.g., implantation of a second conductivity type dopant) which forms the source/drain extensions. However, the high energy process that is used to implant the halo dopants to the correct depth also results in a significant amount of diffusion of that halo dopant towards the wafer substrate and towards channel region. Because the halo dopant is not confined, the punch through reduction function of the halo is minimized. Therefore, there is a need in the art for an improved MOSFET device with sharply defined halos that have a high dopant concentration and that are bound to sharply defined source/drain extensions.